The present invention relates to a hybrid electric vehicle having an electric motor (s) and an internal combustion (IC) engine and a method of control thereof. More particularly, the present invention provides the hybrid electric vehicle with an accelerator pedal which commands torque from either the IC engine or the electric motor in a manner which is essentially non-perceptible to the driver from the operation of an accelerator pedal on a conventional vehicle powered by an IC engine.
The primary objective of the automobile industry is the development of safe vehicles for personal mobility that meet or exceed customer expectations for performance, including acceleration, braking, maneuverability, and comfort, while minimizing the impact on the environment.
The automobile is an integration of many complex nonlinear systems, one of which is the powertrain system. A conventional vehicle powertrain consists of an IC engine, transmission, and driveline including a differential and axle system(s) with drive wheels. An electric vehicle powertrain consists of an electric motor, gearing, and driveline including a differential and axle system with drive wheels. Also included are accessories and peripherals connected to the powerplant such as power steering, power brakes, and air conditioning. The vehicle powertrain is a composition of electrical, mechanical, chemical, and thermodynamic devices connected as a nonlinear dynamic integrated system, with the primary objective of providing the power source for transportation.
Essential to the control of any vehicle is the accelerator pedal. The accelerator pedal does not directly control velocity but rather controls a torque demand to the vehicle power train. Accordingly, when the driver of the vehicle wishes to increase their velocity, the accelerator pedal is actuated to place a torque demand upon the vehicle power train. A torque response to the torque demand is a function of many different variables. For a conventional automotive vehicle powered by an IC engine, torque output at the wheels of the vehicle is related to gear ratios of the transmission and the transaxle; engine RPM, engine compression ratio; throttle setting, intake air temperature; emission system performance; valve operation; and ignition system performance. The engine and drive train controllers accommodate the various variables such that the torque output to the driver is mainly a function of a tactile experience foot maneuvering of the accelerator pedal.
The need to reduce fuel consumption and emissions in automobiles and other vehicles predominantly powered by IC engines is well known. Vehicles powered by electric motors attempt to address these needs. Another alternative solution is to combine a smaller IC engine with an electric motor or motors into one vehicle. Such a vehicle combines the advantages of an IC engine vehicle and an electric vehicle and is typically called a hybrid electric vehicle (HEV). See generally, U.S. Pat. No. 5,343,970 (Severinsky).
HEVs have been described in a variety of configurations. Many HEV patents disclose systems where an operator is required to select between an electric and IC operation. In other configurations, the electric motor drives one set of wheels and the IC engine drives a different set of wheels.
Other HEV configurations have been provided wherein the internal combustion engine and the electric motor power a common drive axle. Some configurations wherein the electric motor and IC engine power a common drive axle are referred to as parallel hybrid electric vehicle (PHEV) configurations. One PHEV configuration has an engine and two traction motors utilized to power a common drive axle and the power train of the system has both the engine and the motors on a common side of the differential for the drive axle.
In another configuration commonly referred to as a post-transmission design, an IC engine is connected with a transmission and differential via an engine clutch. An electric motor is torsionally connected with the differential by a separate motor clutch. The post-transmission parallel hybrid power train accordingly can be powered exclusively by the engine or the electric motor or by both power sources simultaneously.
A vehicle that provides torque to a common or different drive axles through two power sources must be able to partition the torque to the two power sources such that fuel economy and emissions are optimized. In addition, the distribution of torque must be invisible to the driver. The driver commands torque through the accelerator pedal and this amount must be determined. It is desirable that this determined torque request be distributed to the power sources in such a way that the car always behaves in a same manner. However, the controller determines to demand torque from the engine or the motor based upon 20 or 30 operational parameters many of which are non-linear. The controller must consider how to distribute torque in order to maximize fuel economy, extend battery life and range, minimize vehicle emissions and at the same time provide an acceptable driving performance for the vehicle. Many of these factors that are considered by the engine controller are non-linear with respect to the torque demanded at the drive axle.
Further complicating the torque distribution matter is the fact that the amount of torque available from the electric motor is a function of the state of charge (SOC) of the HEV""s batteries. If the HEV battery has a low state of charge, the torque available from the motor will be low. Conversely, if battery charge is high, torque demand from the motor may be at its maximum. U.S. patents discussing these and other issues related to HEV torque output are U.S. Pat. Nos. 5,549,172; 5,899,286; 5,935,040; and 6,064,934.
Experience has shown that in most situations it is preferable to start an HEV forward from a rest position utilizing the electric motor. Electric motors differ from IC engines in that their maximum torque output is essentially available from a rest position, unlike an IC engine which must reach a predefined high RPM output. When the power demand upon the vehicle reaches a certain level, it is usually preferable to thereafter rely upon torque generation from the IC engine. Periods of braking the vehicle allow the vehicle to charge the batteries using regenerative braking. When traveling at highway speeds and attempting to pass another vehicle where wide open throttle conditions exist, typically both power plants will be run to their maximum capacities.
Considering the aforementioned factors, further complicated by switching gear ratios and other operating conditions, it is essential that the torque output or pedal feel at the accelerator pedal be as constant as possible so that the operator of the vehicle can drive the vehicle with confidence in a manner that he or she is used to in driving conventional vehicles powered by an IC engine alone.
Accordingly, it is desirable to provide a HEV which can be powered at various times by the motor, IC engine alone or with the use of both power plants while at the same time providing a constant pedal feel.
To make manifest the above noted desire, a revelation of the present invention is brought forth. In a preferred embodiment the present invention provides a HEV and method of operation thereof where the vehicle is powered by an electric motor and an IC engine. The vehicle is initially powered by the electric motor up to a first vehicle operational parameter level. Typically, the vehicle operational parameter level will be a combination of variables highly dependent upon the power level of the vehicle. Above the first vehicle operation parameter level, the vehicle will be powered by an IC engine. At the time of the transition between the electric motor and the IC engine, a determination will be made of the torque level of the motor. Another determination will be made of the accelerator travel position. A predefined percentage of a maximum IC engine torsional output will be fixed to a predefined accelerator pedal travel second position. Typically, the predefined percentage of maximum engine torsional output will be 100% and the predefined accelerator pedal travel second position will be between 75 and 85% and is commonly placed at the 80% position. The accelerator pedal travel second position is often referred to as a tip in value of the accelerator pedal. The vehicle controller will scale the accelerator travel by a predefined functional relationship from the accelerator pedal travel first position to the accelerator pedal travel second position. In most instances, the predefined functional relationship will be linear. Accordingly, and in most instances, the accelerator pedal travel will be scaled such that at 80% of travel, maximum engine torque will be demanded via the accelerator pedal. Torque demand beyond the 80%, sometimes referred to as boost torque position, flooring the pedal or wide open throttling position of the accelerator pedal, will cause the electric motor to additionally provide torque to the drive axle typically in a preferred linear manner.
It is an advantage of the present invention to provide a HEV which distributes torque to an IC engine and electric motor in a manner which causes operation of the vehicle to be similar to that of a conventional IC engine powered vehicle by use of an accelerator pedal.
Other advantages of the invention will become more apparent to those skilled in the art upon a reading of the following detailed description and upon reference to the drawings.